Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a low frequency alternating current. In an off-line switching power supply an alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. The switched mode power supply controller usually provides output regulation by sensing the output and controlling it in a closed loop. Some off-line switching power supplies include a power factor correction (PFC) feature where the controller provides an internal current loop, as well, to control current shaping of the line current. Operation of a switched mode power supply includes high frequency switching to provide a desired output by varying the on-time, off-time or switching frequency of the switch. Typically, in pulse width modulation PWM, duty cycle is controlled, where the duty cycle is the ratio of the on-time to the total switching period.
Requirements, such as efficiency, size, weight and cost are usually taken into account when designing a switched mode power supply. A controller that controls the switching of the switched mode power supply may be designed to comply with power factor and efficiency requirements of certain regulatory agencies. For example, a conventional controller may be designed to control the switching of the switched mode power supply to provide power factor (PF) and efficiencies higher than predefined values at different load levels and at different conditions of operation (e.g., low line or high line voltages). Higher PF provides sinusoidal input current that is in phase with input voltage and contains acceptable level of low frequency harmonics (e.g., low level of Total Harmonic Distortion THD). High efficiency is also required to reduce power loss and heating at high loads while providing an efficient operation at lower loads and at no load.
In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.